Screen and process for paper patterning

ABSTRACT

A screen (i) for use in forming a patterned paper by a wet-laying technique. The screen is of a mesh material ( 2 ) that is pervious to water and has formed therein repeats of pattern forming elements  3   a–c,    4   a–d  and  5  which are defined by an area of at least partial blockage of the mesh and which are bounded by mesh that is not blocked to drainage. The pattern forming elements have a maximum are of 100 mm 2  that is blocked to drainage. The screen may be used for producing patterned tissue paper from which tea and coffee bags may be produced.

The present invention relates to a screen for use in forming a patternedpaper, particularly but not exclusively a tissue for manufacture of abeverage infusion package, and also to a method of producing patternedpaper.

Beverage infusion packages comprise a beverage precursor material (e.g.tea leaves or ground coffee) enclosed within a bag, pouch, sachet or thelike (all conveniently referred to herein as a bag) of a paper usuallyhaving a basis weight in the range 10 to 30 gm⁻². The paper isfrequently referred to as “tissue” or “tea bag tissue” and is typicallyformed by a conventional wet-laying technique in which an aqueoussuspension of paper forming fibres is laid onto a travelling, waterpervious paper-forming screen with water then being drained through thescreen to produce the paper.

The tissue may be of the heat seal type which incorporates thermoplasticfibres and from which the beverage infusion bag is produced by heatsealing two layers of the tissue together. Alternatively the tissue maybe of the non-heat seal type from which the closure seams(s) of thebeverage infusion bag is/are produced by a mechanical, e.g. crimping,action without heat sealing.

It is often required that the tissue be produced with a pattern. Variousexamples of patterns are used, e.g. the initials and/or logo of themanufacturer of the beverage infusion packages. Alternatively thepattern may comprise a repeat of small circular or diamond-shaped “dots”that are intended to give the impression of perforations in the tissue.

Various methods of producing patterns in tissue are known.

One such technique is fluid jet-patterning. In this technique, thepattern is formed using fluid jets (usually water) directed at the webwhilst it is still on the paper forming screen during the process ofmanufacture by wet-laying. In more detail, a cylindrical patterningscreen rotating about a horizontal axis is provided above the paperforming screen and its wall is pierced by apertures that define thepattern to be produced. Within the cylindrical patterning screen is asource of fluid jet pressure (e.g. a water supply) which is directedradially outwardly so as to traverse the apertures in the patterningscreen and issue as jets which form the pattern in the web. Thistechnique does however have the disadvantage that there can be areduction in the mechanical strength of the web/as compared to thatobtainable without fluid jet patterning) due to disruption of the fibrestructure of the web. In extreme cases the web may be ripped.Additionally the sifting characteristics of the web may be poor allowingfines of beverage precursor material to be lost from the final infusionbag. Moreover water jet patterning requires large amounts of water overand above those used for forming the wet laid suspension in the paperforming screen.

In the case of a pattern which is comprised of repeating dots (to givethe impression of perforations) this may be produced by protrubences(so-called “knuckles”) projecting from the paper-forming screen of a wetlaid manufacturing process. The fibre density (i.e. the number of fibresper unit area) in the region of the paper formed on the knuckle is lessthan in the other regions thereby giving the impression of a pattern.This technique does however suffer the disadvantage that the localisedopen area and reduction in fibre density created by the wire knucklescauses

-   (i) an area of mechanical weakness in the paper web at that point;-   (ii) poor sifting characteristics; and-   (iii) poor and variable pattern definition.

Furthermore the use of “knuckles” limits the morphology of fibres thatmay be used due to the sheet release characteristics of the wire design.A further disadvantage is that there is only limited scope for“personalising” papers (e.g. with a particular logo or other trademark).

Various other methods are known in which the paper forming screen is“configured” to provide desired “effects” in the finally produced paper.

Thus, for example, GB-A-1 008 703 (Crompton) discloses a method ofmanufacturing a heat seal tissue in which the heat seal fibres arepreferentially deposited on a criss-cross network of lines along whichthe heat sealed seams of the final infusion bags will be formed. Moreparticularly, GB-A-1 008 703 discloses a paper forming screen having arepeating pattern (in both the travelling direction of the screen duringpaper manufacture and the direction perpendicular thereto) of spacedregions (e.g. squares) that are blocked to the passage of water, e.g. bygelatin. These spaced, blocked regions define, in effect, a criss-crossarrangement of lines (by which the blocked regions are separated) andthese lines are unblocked. Thus the screen comprises, in effect, acriss-cross arrangement of lines in which there is no blockage todrainage with these lines bounding the discrete blocked-off areas. As aresult of this arrangement, there is preferential accumulation of fibres(including heat seal fibres) along the unblocked lines of the screen dueto the fact that drainage can only occur through these areas. Thecriss-cross lines of the paper (which there is preferential accumulationof fibres) are used for providing the seams of beverage infusion bagswhereas those regions of the paper corresponding to the blocked offareas of the screen provide the “faces” of the bags. Thus the blockedoff areas of the screen typically have an area of at least 500 mm², e.g.about 625 mm².

A further type of paper-forming screen that is “configured” to provide adesired “effect” in the finally produced paper is disclosed in EP-A-0135 231 (Procter & Gamble). The screen of this disclosure comprises ahoneycomb-type frame formed on the base material of the paper formingscreen, this frame providing areas of the screen that are blocked todrainage. The screen is used to produce a paper (such for paper towelsor facial tissue) having surface protrubrunces formed in the open cellsof the honeycomb.

It is an object of the present invention to obviate or mitigate theabovmentioned disadvantages.

According to a first aspect of the present invention there is provided ascreen for use in forming a patterned paper by a wet-laying technique,the screen being of a mesh material that is pervious to water and havingformed therein repeats of pattern forming elements which are defined byan area of at least partial blockage of the mesh and which are boundedby mesh that is not blocked to drainage wherein the pattern formingelements have a maximum area of 100 mm² that is blocked to drainage.

The invention also provides, in a second aspect, a method of forming apatterned paper comprising wet-laying a suspension of paper-formingfibres onto a screen as defined in the previous paragraph so as to forma web and draining water from the web to produce the patterned paper.

In the context of the present invention, a pattern forming element is anarea of the mesh material which is completely bounded externally (andpossibly also internally see infra by unblocked mesh and which (i.e. thepattern forming element) is at least partially blocked to drainage,provided that within the area of the pattern forming element there is nomore than 100 mm² blockage of the screen. The area of the patternforming element that is blocked to drainage is preferably a maximum of50 mm² and may be considerably less, e.g. a maximum of 25 mm² or even amaximum of 10 mm².

When the screen is used for producing paper by a wet-laying technique, apattern is formed in the paper due to the provision in the screen of thepattern forming elements described above. In more detail, water from thefibrous suspension of papermaking fibres laid onto the screen is onlyable to drain through those regions of the screen that are not blockedto drainage. As a result, there is in the final paper a higher fibreconcentration in those areas of the paper corresponding with unblockedareas of the screen as compared to the blocked areas. Since the area ofthe pattern forming element is at least partially blocked to drainagethere is a lower fibre concentration in the areas of the papercorresponding to the pattern forming elements than in the areas betweenthese elements. This lower concentration of fibres gives a visuallydiscernible pattern (corresponding with the original pattern formingelements) in the final paper.

As indicated, individual pattern forming elements are bounded byunblocked mesh. This will be an unblocked area of the mesh surroundingthe outer boundary of the element so that individual pattern formingelements will be separated from each other by an area of the screenwhere there is no blockage to drainage. It should also be understoodthat a pattern forming element can also be bounded internally by an areaof the screen that is unblocked to drainage. The boundary (inner orouter) of the character that is unblocked to drainage will generallycomprise at least one line of apertures of the mesh that are totallyunblocked to drainage. Thus, at a minimum, there will generally be asingle line of (totally) unblocked apertures that may be traced betweentwo adjacent pattern forming elements. Generally the boundary betweentwo adjacent pattern defining elements will comprise (in going from onepattern forming element to the adjacent element) a plurality ofapertures of the mesh that are totally unblocked to drainage.

On the basis of the definitions given in the previous paragraph thefollowing examples illustrate what is intended by the term “patternforming element”.

(a) The letter “I” in which the “bars” and the “stem” of the latter aredefined by a “connected” area of mesh that is blocked to drainage is onepattern forming element;

-   (b) The letter “i” in which the “stem” and the “dot” are separate    areas of mesh that are blocked to drainage comprises two pattern    forming elements even though it is regarded as a single character.-   (c) The letter “T” in which the “bar” and the “stem” are areas of    the mesh at least partially blocked to drainage but with a slight    separation between the “bar” and “stem” is regarded as two pattern    defining elements. The extent to which the “bar” and “stem” are    separated could be relatively small so that in the final patterned    paper the separation is not noticed with normal vision.

(d) The letter “O” in which the outline of the letter (i.e. the“circular line”) would be regarded as one pattern defining element(because the “circular line” is bounded both internally and externallyby unblocked regions of the mesh)

(e) Following on from (d), a “motif” comprised of, say, a letter withina circular (or oval, elliptical etc) boundary comprises two patternforming elements, i.e. the letter and the circular boundary.

The use of pattern forming elements in which the area that is blocked todrainage does not exceed 100 mm² ensures that the individual patterns inthe web do not provided any significant weakness therefor.

The pattern forming elements may be of any desired visual appearance andmay for example, be letters of the alphabet or alternatively may bepictorial. It is possible for the pattern forming elements as definedabove to be combined with non-blocked areas of the web so that it is thecombination which gives rise to a recognisable pattern. Thus considerfor example the letter “O”. In this case, the pattern forming elementwill be formed by blockage of the screen to define the outline of theletter whereas the centre of the letter will be provided by an unblockedregion of the screen.

The pattern forming elements are formed by at least partial blockage ofthe apertures of the mesh in the area of the pattern forming element.The pattern forming elements may for example be defined by areas ofcomplete blockage of the web to drainage. Alternatively the patternforming elements may incorporate areas where there is no blockage todrainage. Such unblocked areas may comprise one or more full aperturesof the mesh that is/are unblocked to drainage. Several such unblockedfull apertures of the mesh maybe juxtaposed to each other. Alternativelyor additionally unblocked areas of the pattern forming elements may beformed by incomplete blockage of individual apertures of the mesh.Several such partially blocked apertures may be provided adjacent toeach other so that their individual unblocked areas lie within theboundary of a larger unblocked area.

Within any one pattern forming element there may be severaldiscontinuous areas that are unblocked to drainage.

It is preferred that a minority of the area of the pattern formingelement is open to drainage. This ensures sufficient fibre concentrationwithin the bounds of the pattern to ensure integrity of the web.Generally there will be at least 60% blockage in this area, morepreferably at least 80%. The blockage may for example be 90%–100% of thearea of the pattern forming element.

The screen in accordance with the invention will generally be in theform of an endless belt.

The individual pattern forming elements will generally be repeated inboth the longitudinal transverse directions of the screen.

It is preferred that the material used for effecting blockage of basematerial does not project out of the plane of the base material.

As indicated above, the base material of the paper screen is a mesh-likestructure, most preferably one formed from synthetic monofilaments.Ideally the diameter of the fibres forming the mesh is in the range0.15–0.30 mm. Preferably also the mesh comprises 32 filaments percentimetre in the machine direction and 30 filaments per centimetre inthe cross direction. A suitable mesh material is available from AlbanyInternational under the trade name of MONTOTEX K3.

Blockage of the base material to drainage may be provided by applying asynthetic resin to block apertures of the mesh so that the desiredrepeat of pattern forming elements is produced. Conveniently the resinmay be applied by a printing technique such a screen printing, gravureprinting, blanket offset printing. A further possibility is the use ofphotoresist technology in which a negative of the desired formation ofpattern forming elements is juxtaposed to the base screen materialimpregnated with curable resin. Light (e.g. uv radiation) is then usedto cure those regions of the resin on the screen corresponding to thepattern forming elements and uncured resin removed to leave the finalscreen. Other techniques that can be used include transfer coating.

The polymer used for effecting blockage of the apertures should becompatible with the base material of the screen and may for example be afilm forming polymeric resin. Examples of suitable resins includepolyamides and polyurethanes.

Paper forming screens as described above may be used for producingtissue, for beverage infusion bags, by standard wet-laying techniques,e.g. using an inclined wire paper making machine. The material producedmay be of the heat seal or non-heat seal type and may for example have abasis weight of 10 to 30 gm⁻², e.g. 10 to 20 gm⁻². In papers produced(in accordance with the invention) for beverage infusion bags thespacings between the repeats of the patterns will generally be such thatthere are a number of repeats of the pattern on each “face” of thebeverage infusion bag.

Typically the fibres used in this paper making process will have alength of 3 to 5 mm and may comprise only cellulose fibres (for anon-heat seal material) or a blend of cellulose and thermoplastic fibresfor a heat seal material.

Tissues produced with the paper making screen of the invention have anumber of advantages compared with those produced by fluid jetpatterning. In particular, tissues produced in accordance with theinvention have good mechanical properties (since there has been nodisruption of the fibres by a patterning jet). The improved strength isan advantage for resisting tearing of the web by deckle edge sprays andalso for conversion of the web into beverage infusion packages onstandard conversion machinery. Furthermore the papers have bettersifting properties. A further advantage lies in the fact that productionspeed of the paper web is not limited by the speed by which the fluidpatterning arrangement may be operated.

Similar advantages apply to the present invention as compared to the useof “knuckles” for forming a pattern in the paper. Moreover, as comparedto the use of “knuckles”, the present invention allows completeflexibility of the choice of the pattern forming elements and theinvention is not confined to the use of fibres of a particularmorphology.

Tissues produced in accordance with the invention may be produced onstandard conversion machinery to produce (beverage infusion bags).

The invention will now be described, by way of example only, withreference to accompanying drawings, in which:

FIG. 1 illustrates one embodiment of paper-forming screen in accordancewith the invention;

FIG. 2 illustrates, to a much enlarged scale, the base material fromwhich the screen illustrated in FIG. 1 is produced;

FIG. 3 illustrates the manner in which a pattern is provided in the basematerial of FIG. 2 to produce a screen as shown in FIG. 1; and

FIG. 4 illustrates the production of paper.

Referring to FIG. 1, there is illustrated a portion of a screen 1 inaccordance with the invention for use in forming, by a conventionalwet-laying technique, a patterned tissue for the production of beverageinfusion bags such as tea-bags or coffee-bags. The screen 1 is formed ofa water pervious base material 2 (described below with reference to FIG.2) in which regions thereof have been at least partially blocked, in themanner described more fully below, to the passage of water so as todefine repeats of pattern forming elements. For convenience, the atleast partially blocked regions are depicted in FIG. 1 by darkcoloration and unblocked areas of the base material 2 are depicted aswhite.

Various types of pattern forming element are shown in FIG. 1 with eachtype repeating in two perpendicular directions, i.e. along the lengthand across the width of the screen. The repeating pattern formingelements are as follows:

(a) three pattern forming elements 3 a, 3 b, 3 c comprised of theletters J, R and C on a “lozenge-shaped” background, each of the lettersJ, R and C respectively being defined by partially blocked regions ofthe base material and the “lozenge-shaped” being defined by an unblockedregion;

(b) pattern forming elements 4 a, 4 b, 4 c and 4 d which together definea representation of a cup and saucer; and

(c) pattern forming elements 5 comprised of small diamonds (representedin FIG. 1 by the black, diamond shaped dots).

Each of the pattern forming elements 3 a–c, 4 a–d and 5 has an area ofless than 100 mm².

The base material 2 is shown to a much enlarged scale in FIG. 2 and isof a mesh-like structure comprised of synthetic plastics monofilaments 6arranged to define apertures 7 whereby material 2 is water pervious. Itshould be appreciated that those areas of the screen 1 which are shownin FIG. 1 as white (to represent unblocked areas) are of the open meshstructure shown in FIG. 2.

The manner in which various pattern forming elements are formed is byblockage or partial blockage (to the passage of water) of apertures 7 invarious regions of the material 2 as will now be explained withreference to FIG. 3.

FIG. 3 shows the manner in which the letter “J” is formed for thepatterning of the screen. More particularly, for each repeating “J”there is a “J-shaped” region in which certain of the original apertures7 within the bounds of the “J” are blocked to an extent of 75% by apolymer 8. More particularly, unblocked areas of four apertures 7 arearranged in groups so as to define a larger diamond-shaped unblockedareas of the mesh. Furthermore, as seen in FIG. 3, the unblockeddiamond-shaped areas are arranged such that the “bar” and “verticalstem” of the letter “J” incorporates several such diamond-shaped areaswhere there is no blockage (of the mesh) to drainage.

The letter “C” of the letter set “JRC” may be constructed in like manneras also may be the letter “R” although in the latter case it should benoted that the area of the “loop” of the letter is unblocked to drainageso as to give the required overall appearance.

Each “cup and saucer” design is made up of four pattern forming elements4 a, 4 b, 4 c and 4 d each of which is bounded by an unblocked region ofthe screen and which together with unblocked regions of the mesh givethe overall appearance of a cup-and-saucer. Each of the elements 4 a–dmay be formed in a similar manner to the letter “J” as described above.

The diamonds 5 may be constructed in like manner to the letter “J”.

The screen 1 may be produced, for example, by a printing technique whichlays down onto the base material 2 the areas of polymer (e.g. asrepresented by reference numeral 8 for the letter “J”) which provide forat least partial blockage of the apertures 7 thereby defining thepattern forming elements 3 a–c, 4 a–d and 5. Examples of printingtechniques that can be used include screen printing, gravure printing,blanket offset printing. Transfer coating may also be used.

Alternatively the screen 1 (with its areas of cured resin) may beproduced using photoresist technology. Thus, for example, the basematerial 2 may be coated with a resin (e.g. curable by ultravioletlight) and juxtaposed to a negative of the arrangement of blocked and/orpartially blocked apertures to be provided in the final screen. Theassembly of negative and resin coated base material 2 is then irradiatedso as to cure the resin in those areas of the base material which are tobe blocked or partially blocked. Subsequently uncured resin is removedto leave the final screen. It is preferred, but not essential, that thecurable resin system is one which is such that the uncured areas may beremoved by water.

As indicated above, the screen 1 is intended for use in producing paperby a wet-laying technique. Such a technique is illustrated schematicallyin FIG. 4 from which it will be seen that the screen 1 is provided as anendless belt onto which is deposited an aqueous suspension ofpaper-forming fibres from a headbox 9. As the laid suspension is carriedalong by the screen, water is drained through the screen (e.g. with theaid of vacuum boxes) so as to form the paper web 10 which issubsequently removed for drying and reeling operations, all of which areentirely conventional. Although not specifically illustrated in FIG. 4,the endless belt may be of the “inclined wire” type.

The final web 10 carries a pattern corresponding to that on the screen1. The formation of this pattern results from the drainagecharacteristics of the screen 1. More particularly, during drainage ofthe laid suspension, a greater “concentration” of fibres accumulates inthose areas of the screen that are not obscured by resin than in thoseareas which are so obscured. The visible pattern on the final web istherefore due to differential amounts of fibres in the web as betweenthose areas laid on blocked and unblocked regions of the screen.Consequently the final has a pattern corresponding to that of the screen1. Thus for the illustrated embodiment the paper web 10 incorporatesrepeats of the letter set JRC on a lozenge-shaped background and repeatsof the cup and saucer. There are also repeats of the diamond shape andin the final paper web 1 these give the impression of perforations inthe paper.

1. A method of forming a patterned tissue paper having a basis weight of10 to 30 g m⁻² on an inclined wire paper making machine comprisingwet-laying a suspension of appear-forming fibres onto the inclinedscreen that is pervious to water to form a web and draining water fromthe web to produce the patterned paper wherein the screen is of a meshmaterial that is pervious to water and has formed therein repeats ofpattern forming elements which are defined by an area of blockage of themesh by a material that does not project out of the plane of the meshmaterial and which are bounded by mesh that is not blocked to drainagewherein the pattern forming elements have a maximum area of 50 mm² thatis blocked to drainage and the pattern forming elements incorporateareas where there is no blockage of the web to drainage.
 2. A method asclaimed in claim 1 wherein the pattern forming elements have a maximumarea of 25 mm² blocked to drainage.
 3. A method as claimed in claim 2wherein the pattern forming elements have a maximum area of 10 mm²blocked to drainage.
 4. A method as claimed in claim 1 wherein patternforming elements have at least 60% of their area blocked to drainage. 5.A method as claimed in claim 4 wherein pattern forming elements have atleast 80% of their area blocked to drainage and the remaining area ofthe elements open to drainage.
 6. A method as claimed in claim 1 whereinthe unblocked areas incorporated in the pattern forming element compriseone or more full apertures of the mesh that is/are unblocked todrainage.
 7. A method as claimed in claim 6 wherein a plurality of suchunblocked full apertures of the mesh are juxtaposed to each other.
 8. Amethod as claimed in claim 1 wherein unblocked areas of the patternforming elements are formed by incomplete blockage of individualapertures of the mesh.
 9. A method as claimed in claim 8 wherein aplurality of such partially blocked apertures are provided adjacent toeach other so that their individual unblocked areas lie within theboundary of a larger unblocked area.
 10. A method as claimed in claim 1wherein at least some of the pattern forming elements are letters of thealphabet.
 11. A method as claimed in claim 1 wherein at least some ofthe pattern forming elements are pictorial.
 12. A method as claimed inclaim 1 wherein the pattern forming elements are formed by at leastpartial blockage of apertures of the mesh by means of a polymericmaterial.
 13. A method as claimed in claim 12 wherein the polymericmaterial has been applied by a printing technique.
 14. A method asclaimed in claim 13 wherein the polymeric material blocking theapertures of the mesh comprises a polyamide or polyurethane.
 15. Amethod as claimed in claim 12 wherein the polymeric material has beenapplied by a photoresist technique.
 16. A method as claimed in claim 15wherein the polymeric material blocking the apertures of the meshcomprises a polyamide or polyurethane.
 17. A method as claimed in claim1 wherein the tissue has a basis weight of 10 to 20 g m⁻².
 18. A methodas claimed in claim 1 wherein the tissue is in the form of a heatsealable material.
 19. A beverage infusion package formed of the paperproduced by the method as claimed in claim
 18. 20. A method as claimedin claim 1 wherein the tissue is in the form of a non-heat sealablematerial.
 21. A beverage infusion package formed of the paper producedby the method as claimed in claim 20.